AMSOIL 10W-40 100% SYNTHETIC MARINE ENGINE OIL

AMSOIL 10W-40 100% Synthetic Marine Engine Oil delivers serious wear protection and rust prevention for four-stroke marine engines that spend their lives in one of the harshest operating environments any powerplant can face. Built for anglers, boaters, and anyone running a four-stroke outboard, inboard, sterndrive, or personal watercraft who refuses to cut corners on engine protection.

Marine engines deal with a unique combination of abuse. Sustained high-rpm operation, heavy loads, saltwater or brackish environments, and long stretches of inactivity between seasons all conspire to shorten engine life. The humidity and moisture exposure alone can corrode internal components during storage, and many conventional oils simply aren't formulated to handle the thermal stress of wide-open throttle runs combined with months of sitting idle on a trailer or in a slip.

FIGHTS ENGINE WEAR AT HIGH RPM
Marine four-strokes routinely operate at sustained high RPM that would be unusual in automotive applications. That kind of sustained output generates extreme heat and mechanical shear forces that thin out lesser oils, reducing their ability to maintain a protective film on critical engine components. AMSOIL 10W-40 Synthetic Marine Engine Oil uses naturally strong synthetic base oils that resist viscosity loss under shear. It recorded a high-temperature/high-shear viscosity of 4.2 cP in ASTM D5481 testing, confirming film strength where it counts most, under real operating heat and stress. In Four-Ball Wear testing per ASTM D4172, it produced a wear scar of just 0.40 mm, demonstrating the effectiveness of its robust anti-wear additive package in metal-to-metal contact zones. That translates directly to compression retention, sustained power output, and longer engine life in outboards like the Yamaha F150, Mercury FourStroke 200, Honda BF250, and Suzuki DF300.

EXCELLENT RUST AND CORROSION PROTECTION
Rust protection is not something motor oil does naturally. It has to be engineered in. AMSOIL Synthetic Marine Engine Oil is heavily fortified with corrosion inhibitors designed to coat and protect internal engine surfaces both during operation and during extended storage. Whether your boat sits on a lift in a coastal marina all summer or gets winterized and parked for six months, this oil actively guards against the moisture-driven corrosion that eats away at bearings, cylinder walls, and valve train components. This is especially critical for saltwater boaters running Honda outboards, Yamaha VMAX SHO engines, Mercury Verado powerheads, or MerCruiser inboards where corrosive exposure is constant.

LOW VOLATILITY FOR THERMAL STABILITY
High operating temperatures cause lesser oils to volatilize, literally burning off the lighter molecules and leaving behind a thicker, less effective lubricant. AMSOIL 10W-40 Marine Oil posted a Noack Volatility result of just 7.5% weight loss in ASTM D5800 testing, meaning it holds together and stays in grade even under severe thermal loads. A flash point of 240°C (464°F) further confirms the thermal resilience of this formulation. For supercharged marine engines and high-output applications where underhood temperatures climb fast, that stability makes a measurable difference.

NMMA FC-W CATALYST COMPATIBLE CERTIFIED
This oil carries NMMA FC-W Catalyst Compatible certification (Registration Number FC-00850T), which is backward-compatible with the standard FC-W specification. That certification matters because it confirms the oil meets the stringent performance requirements set by the National Marine Manufacturers Association for use in modern four-stroke outboard motors, including those equipped with catalytic converters. It also meets API SM and JASO MA specifications.

BROAD FOUR-STROKE MARINE APPLICATION COVERAGE
AMSOIL 10W-40 Synthetic Marine Engine Oil is designed for use in gasoline-fueled four-stroke inboards, outboards, inboard/outboards, supercharged watercraft engines, and personal watercraft. Compatible applications include engines made by Honda, Mercury, Yamaha, Johnson/Evinrude, Bombardier/BRP, Suzuki, Nissan, Tohatsu, OMC, Volvo-Penta, MerCruiser, Chevrolet, and Ford. It works as a direct high-performance replacement for OEM-branded oils and is Warranty Secure, keeping your factory warranty intact.

SERVICE LIFE
For four-stroke outboard motors and personal watercraft, change oil at the engine or watercraft manufacturer-recommended interval due to the potential for fuel dilution or water contamination. For inboard and I/O applications, AMSOIL supports oil changes at two times the engine manufacturer-recommended interval or one year, whichever comes first. Compatible with other synthetic and conventional motor oils, though mixing will reduce the performance benefits and service life of the AMSOIL product.